Injury-free priming: induction of Primary Afferent Collateral Sprouting in uninjured sensory neurons in vivo primes them for enhanced axon outgrowth in vitro.

Prior 9conditioning9 nerve lesions can prime DRG neurons for enhanced axon regeneration. We tested the hypothesis that adult DRG neurons can be similarly primed for axon elongation without axonal injury through induction of Primary Afferent Collateral Sprouting (PACS). PACS was induced in uninjured T11 DRG neurons by denervating dermatomes rostrally and caudally. In this model uninjured adult DRG neurons extend uninjured axons into denervated skin in vivo. Collateral sprouting by the spared T11 DRG neurons was confirmed by mapping the expansion of its responsive zones of skin up to 14 days. At 7 or 14 days after induction of PACS, DRG neurons were dissociated and cultured without growth factors for 18 hours. Neurons from T11 PACS DRG had longer mean neurite lengths than neurons from naive DRG. A larger proportion of neurons from T11 PACS DRG showed an elongating or arborizing phenotype than neurons from naive DRG. DRGs serving nerves transected to create the denervated dermatomes contained large numbers of neurons expressing Activating Transcription Factor 3 (ATF3), a marker of the injury response. In contrast, T11 DRG from both unoperated rats and PACS rats contained very few neurons expressing ATF3. Microarray analyses and qRTPCR of T11 DRG and denervated/reinnervated skin reveal regulation of receptor/ligand systems as well as additional potential regulators of growth during PACS. We suggest that extracellular cues in denervated skin modify the intrinsic growth program of uninjured DRG neurons that enhances their ability to elongate or arborize even after explantation. Collectively, these data indicate that induction of PACS does not induce an injury response, yet primes many of these uninjured neurons for in vitro axon growth.